Portable object having multi-level demodulation and being inductively coupled to a fixed station

ABSTRACT

The portable object ( 1 ) is provided with an antenna ( 3 ) inductively coupled to a fixed station of a remote transmission device that emits an alternating magnetic field whose amplitude is modulated on 2&lt;SP&gt;N&lt;/SP&gt; levels by data, which are to be transmitted and are encoded in N bits. The portable object has a variable load impedance (Z) connected to antenna terminals ( 3 ), and a control loop for controlling the voltage (Vac) to the terminals of the load impedance. The control loop comprises, in series-connected manner, a rectifier circuit ( 5 ) for rectifying the voltage (Vac) to the antenna terminals, an analog/digital converter ( 9 ) having n bits and being clocked at an oversampling frequency (Ck) much higher than the frequency of the data, and comprises a control circuit ( 10 ) serving to modify the load impedance (Z) according to the output voltage (Vdc) of the rectifier circuit ( 5 ). A digital processing circuit ( 11 ) connected to the output of the converter ( 9 ) furnishes the demodulated data in the form of N-bit words.

BACKGROUND OF THE INVENTION

The invention relates to a portable object provided with an antennainductively coupled to a fixed station of a remote transmission device,the portable object comprising a variable load impedance, connected toterminals of the antenna, and a control loop for regulating the voltageat terminals of the load impedance comprising rectifier means forrectifying the voltage at the terminals of the antenna and control meansdesigned to modify the load impedance according to the output voltage ofthe rectifier means.

STATE OF THE ART

As represented in FIG. 1, inductive coupling is conventionally used forremote transmission of data between a portable object 1, in particularof the type constituted by smart cards, tickets, contact-less labels,known under the name of RFID (radiofrequency identification) labels,etc., and a fixed station 2, for example constituted by a card reader, aRFID reader, etc., in the field of object identification, accesscontrol, remote toll payment, etc.

The document WO-A-00/63830 describes a fixed station coupled to astandard passive RFID label with variable load impedance.

In most remote transmission devices of this type, the portable object 1is passive. It is remote supplied with power by the fixed station 2,which comprises its own power supply circuit, by means of the inductivecoupling which is achieved by antennas 3 and 4, formed by coils. Thesedevices generally use the load modulation principle.

Transmission of binary data from the fixed station 2 to the portableobject 1 is conventionally performed, in particular in the documentWO-A-00/03352, by amplitude shift keying (ASK). As represented in FIG.2, the amplitude of the magnetic field H emitted by the fixed station 2and received by the portable object 1 takes a first value when the datasignal to be transmitted takes the binary value 0 and a second value,lower than the first one in FIG. 2, when the data signal to betransmitted takes the binary value 1.

The antenna 3 of the portable object 1 is the seat of an electromotiveforce which creates an AC voltage Vac, at the terminals of the antenna3, the amplitude of which voltage depends, among other things, on themagnetic field emitted by the fixed station, on the load impedance Zconnected in parallel to the antenna 3 and on the distance between theportable object and the fixed station. To detect this amplitude and toenable remote power supply of the portable object 1, the lattercomprises a rectifier circuit 5 connected to the output of the antenna3, as represented in FIG. 1. A DC voltage Vdc, the amplitude whereof isrepresentative of the transmitted data, is therefore produced at theoutput of the rectifier circuit 5, from the voltage at the terminals ofthe coil of the antenna 3, induced by the magnetic field emitted by thereader 2.

The rectifier circuit 5 can be formed by any suitable circuit enablingan AC voltage Vac to be transformed into a DC voltage Vdc. It can forexample be formed by one of the circuits represented in FIGS. 3 to 6 andconventionally comprising, respectively, a diode (FIG. 3), a diodebridge (FIG. 4) or a half-wave rectifier circuit (FIG. 5) or a full-waverectifier circuit (FIG. 6) using MOS type transistors.

The magnetic field seen by the portable object 1 varies rapidly with thedistance separating the portable object from the reader 2. Theelectromotive force induced in the antenna 3 of the portable object 1can thus vary in large proportions. As the portable object 1 isessentially constituted by an integrated circuit connected to theantenna 3, this circuit is generally protected by means for limiting thevoltage at the terminals of the antenna, thus enabling standardlow-voltage technologies to be used, which are less expensive thanhigh-voltage technologies. It is known to use for this a shunt-typeregulation circuit the principle whereof is illustrated in FIG. 1. Theregulation circuit of FIG. 1 comprises a regulator 6 connected to theoutput of the rectifier circuit 5 and controlling the value of the loadimpedance Z, connected to the terminals of the antenna 3, according tothe output DC voltage Vdc of the rectifier circuit 5.

In a first embodiment of the regulator 6, illustrated in FIG. 7, the DCvoltage Vdc is applied via a resistor R1 to the source of a PMOStransistor T, having its drain grounded via a resistor R2, while areference voltage Vref is applied to the gate of the transistor T. Theregulator thus supplies a control voltage Vc designed to control thevalue of the variable load impedance Z. The PMOS transistor can bereplaced by a bipolar transistor of PNP type, or by a JFET typetransistor.

In the alternative embodiment illustrated in FIG. 8, the regulator 6comprises a divider bridge formed by two resistors, R3 and R4, connectedin series between the output of the rectifier circuit 5 and ground, andan amplifier 7 having an input (positive in the figure) connected to themid-point of the divider bridge and another input (negative in thefigure) connected to the reference voltage Vref.

The variable load impedance Z can be formed by a transistor T, forexample of MOSFET, JFET or bipolar type, having a control electrodeconnected to the output of the regulator 6. The voltage Vdc is thusregulated to a value dependent on the reference voltage Vref, whichregulates and limits the voltage at the terminals of the antenna 3. Thevariable impedance can also be formed by any other known means, forexample by a bundle of resistors to be connected or disconnectedselectively from the antenna 3 according to the value of the DC voltageVdc.

Numerous potential applications, for example biometric identification,require a large quantity of data to be transferred in a time that has toremain sufficiently low for a user to consider that transmission isalmost instantaneous. An increase of the frequency of the binary datasignal could enable the binary transmission rate to be increased.However, such an increase would have the consequence of increasing thenecessary passband. Increasing the passband would however require thequality factor of the label to be reduced, to the detriment of theremote power supply performances.

OBJECT OF THE INVENTION

The object of the invention is to provide a portable object of a remotetransmission device by inductive coupling enabling the transmission rateof the data to be transmitted between the fixed station and the portableobject to be increased and not presenting these shortcomings.

According to the invention, this object is achieved by a portable objectaccording to the appended claims and, more particularly by the fact thatthe fixed station emitting an alternating magnetic field modulated inamplitude on 2^(N) levels by data to be transmitted, encoded on N bits,the portable object comprises demodulation means comprising an n-bitanalog-to-digital converter, clocked at an oversampling frequency muchhigher than the frequency of the data and connected between therectifier means and the control means, and digital processing meansconnected to the output of the converter.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features will become more clearly apparent from thefollowing description of particular embodiments of the invention givenas non-restrictive examples only and represented in the accompanyingdrawings, in which:

FIG. 1 schematically illustrates a remote transmission device withinductive coupling according to the prior art.

FIG. 2 represents the variations of the magnetic field H, emitted by thefixed station, versus the value of the binary data signal to betransmitted to the portable object of a device according to FIG. 1.

FIGS. 3 to 6 represent various alternative embodiments of a rectifiercircuit of a device according to the prior art.

FIGS. 7 and 8 illustrate two alternative embodiments of a regulator of adevice according to the prior art.

FIG. 9 represents different signals obtained in a device according toFIG. 1 using a 4 levels modulation.

FIG. 10 represents a device enabling multi-level demodulation.

FIG. 11 represents a device according to the invention.

FIG. 12 represents different signals obtained in a device according toFIG. 11.

FIG. 13 illustrates an alternative embodiment of a device according tothe invention.

DESCRIPTION OF PARTICULAR EMBODIMENTS

The device according to the invention uses multi-level modulation. Thus,the amplitude of the alternating magnetic field H emitted by the fixedstation 2 is modulated on 2^(N) levels by data to be transmitted,encoded on N bits, with N≧1. In the particular embodiment represented inFIG. 9, the data are encoded in the form of 2-bit words (N=2) and theamplitude of the emitted magnetic field can thus take 4 different valuesrespectively corresponding to the different values (00, 01, 10 and 11)of the data to be transmitted. In FIG. 9, the emitted field has aminimum amplitude to transmit a datum 00 (between the times t1 and t3),then respectively takes higher first and second intermediate values torespectively transmit the data 01 (between the times t3 and t5) and 10(between the times t5 and t7), and a maximum value to transmit the datum11 (between the times t0 and t1 and between the times t7 and t9).

In a portable object 1 according to FIG. 1, comprising a regulationloop, the change of level of the amplitude of the emitted field causes achange of level of the electromotive force and consequently a fastvariation (decreasing at the time t1 and increasing at the times t3, t5and t7) of the amplitude of the AC voltage Vac generated at theterminals of the antenna 3 and of the load impedance Z. The regulationloop reacts to this variation by modifying the control voltage Vc of theimpedance Z to bring the amplitude of the voltage Vac back to a presetregulated value after a transient period having a duration (t1-t2,t3-t4, t5-t6 or t7-t8) which depends on the passband of the regulationloop.

With this type of regulation loop, the level and variations of the ACvoltage Vac are not representative of the levels of the emitted magneticfield, corresponding to the binary digital data to be transmitted. Thisis more particularly obvious between the times t3 and t9, correspondingto increasing values of the level of amplitude of the emitted magneticfield, for which the passage from one level to the immediately higherlevel causes the same variations of the voltage Vac. However, asrepresented in FIG. 9, after the transient period, the amplitude of thecontrol voltage Vc, on output from the regulator 6, is proportional tothe amplitude of the emitted magnetic field. The amplitude of thecontrol voltage Vc is therefore representative of the level of amplitudeof the emitted magnetic field and consequently of the data to betransmitted.

The portable object represented in FIG. 10 uses the control voltage Vcto perform multi-level demodulation of the data transmitted between thefixed station 2 and the portable object. This portable object differsfrom the portable object according to FIG. 1 by the addition of an N-bitanalog-to-digital converter 8 (A/D), having its input connected to theoutput of the regulator 6 and which supplies the demodulated data in theform of N-bit words on output. However, the variations of the controlvoltage Vc are small, in particular when the variable load impedance Zis formed by a MOS type transistor having its gate connected to theoutput of the regulator 6. The control voltage Vc in fact then variesbetween the threshold voltage of the transistor and the thresholdvoltage plus a few hundred millivolts. This imposes a high resolution ofthe analog-to-digital converter 8, which may give rise to problems atthe level of the consumption thereof and of the necessary siliconsurface.

To avoid these problems, the current flowing through the load impedanceZ can be used as a quantity representative of the transmitted data, theanalog-to-digital converter 8 then being connected to the output of acurrent measuring interface. The variations of this current are in factsubstantially greater than those of the control voltage Vc. However,although these large variations constitute an advantage when theportable object 1 is situated at a predetermined distance from the fixedstation 2, this does cause a problem if the portable object has to beable to be used over a wide range of distances. A complete range ofdistances can for example typically correspond to currents ranging froma few microamps to a few tens of milliamps. This very wide rangerequires a very high resolution of the analog-to-digital converter 8and/or addition of a system enabling the problem of the distance betweenthe portable object 1 and the fixed station 2 to be overcome. Such asystem, notably comprising automatic gain control and/or automatic rangeselection circuits, makes the demodulation circuit of the portableobject more complex.

The portable object represented in FIG. 11 enables these drawbacks to beovercome. For this, an n-bit analog-to-digital converter 9 (A/D) isconnected in the regulation loop, between the rectifier circuit 5 and acontrol circuit 10 supplying the control voltage Vc designed to controlthe load impedance Z. In a preferred embodiment, N≧2 and n<N. Theanalog-to-digital converter 9 is preferably a one-bit converter (n=1),which can thus be formed by a simple comparator. This very lowresolution, which enables the number of components to be reduced to theminimum, is compensated by a high time resolution, obtained by clockingthe converter 9 at an oversampling frequency much higher than thefrequency of the data transmitted by the fixed station. The oversamplingfrequency is determined by a clock circuit supplying clock signals Ck toa clock input of the converter 9. For example, for a data frequency ofabout 200 kHz, the oversampling frequency can advantageously becomprised between 10 and 20 MHz. Although the output digital signals ofthe converter 9 contain the necessary information for recovery of thedata transmitted by the fixed station 2, these digital signals are notdirectly usable. The output of the converter 9, connected to the controlcircuit 10, is therefore in addition connected to the input of a digitalprocessing circuit 11 designed to supply the demodulated data on N bits.

Operation of the demodulation circuit of the portable object of FIG. 11is illustrated by the signals represented in FIG. 12, in the case wherethe low-resolution analog-to-digital converter 9, included in theregulation loop of the AC voltage Vac at the terminals of the antenna 3,is formed by a simple comparator (n=1) supplying a sequence of bits atthe oversampling frequency. The control circuit 10 is formed by anintegrator supplying an analog voltage signal Vc representative of then-bit output signals of the converter, for example in the form of arising voltage ramp when the output of the converter 9 is at 1 and of adescending voltage ramp when this output is at zero. The control voltageVc, obtained on output from the control circuit 10, is therefore neverstatic. Thus, after a transient period, when the regulation loop is inan equilibrium position, the control voltage Vc has the form of asaw-tooth signal around a mean level proportional to the level of theelectromotive force, i.e. proportional to the level of the magneticfield emitted by the fixed station 2 and, consequently, representativeof the data transmitted by the fixed station.

In the example illustrated in FIG. 12, the magnetic field H variesbetween the times t0 and t9 in the same way as in FIG. 9. The change oflevel of the amplitude of the emitted magnetic field H causes a changeof level of the electromotive force and, consequently, a rapid variation(decreasing at the time t1 and increasing at the times t3, t5 and t7) ofthe AC voltage Vac generated at the terminals of the antenna 3 and ofthe load impedance Z. The digital output of the converter 9, whichcompares the amplitude of the rectified voltage with a preset threshold,is then formed by a sequence of bits at the oversampling frequency and,more particularly, by an alternation of pairs of 0's and of pairs of1's: 1100110011001100 between the times t0 and t1, 0000001100110011between the times t1 and t3, 1100110011001100 between the times t3 andt5, 1111001100110011 between the times t5 and t7 and 1100110011001100between the times t5 and t7. A decrease of the level of the emittedmagnetic field is therefore expressed (for example at the time t1) byemission of more than two consecutive 0's, thus causing a decrease ofthe mean amplitude of the control voltage Vc. In like manner, anincrease of the level of the magnetic field emitted is expressed (forexample at the times t3, t5 and t7) by emission of more than twoconsecutive 1's, causing an increase of the mean amplitude of thecontrol voltage Vc. The number of consecutive 0's or 1's increases withthe amplitude of the electromotive force jump. The digital signalsoutput from the converter 9 thus contain information representative ofthe sign and of the amplitude of the level variation of theelectromotive force generated by the emitted magnetic field, in otherwords information representative of the derivative of the envelope ofthe electromotive force or of the magnetic field. The digital processingcircuit 11, designed to transform this information into binary digitalsignals representative of the transmitted data, therefore comprises atleast a digital integration function. The integration functions of thecontrol circuit 10 and/or of the digital processing circuit 11 can beperformed by means of integrators and/or low-pass filters.

Thus, in the embodiment of FIG. 11, the regulation loop simultaneouslyperforms a part of the analog-to-digital conversion, which makes thewhole assembly more compact. Moreover the effect of possible slowvariations of the mean magnetic field level, due to a movement of theportable object 1, is attenuated by the fact that the information outputfrom the converter 9 is representative of the derivative of the envelopeof the magnetic field. Slow variations of the mean field level aretherefore treated as noise and do not disturb demodulation. Postponementof part of the processing digitally in the digital processing circuit isalso advantageous.

On the other hand, the load impedance Z is continuously modulated, evenat constant field, and this may disturb communication in the oppositedirection, between the portable object and the fixed station. Thisdrawback can however be overcome by disabling the regulation loop duringemission of data modulated by the portable object.

To further overcome the problems of mean field variations, it ispossible to adapt some characteristics of the control circuit 10, forexample the slope of the ramps of the control voltage Vc, to the meanmagnetic field by means of a passband loop substantially narrower thanthat of the main control loop.

The alternative embodiment of FIG. 13 enables the resolution of thedemodulation to be increased by reducing the influence of thequantification noise generated by the low-resolution analog-to-digitalconverter 9. For this, a sigma-delta type modulator 12 is connectedbetween the input of the analog-to-digital converter 9 and an additionalinput of the digital processing circuit 11. The input of the converter 9(rectified voltage Vdc) is thus converted, by the modulator 12, into anoversampled binary signal and this oversampled binary signal is combinedwith the output of the converter 9 in the digital processing circuit tosupply the demodulated data in the form of N-bit words.

1. Remote transmission device comprising a fixed station and a portableobject provided with an antenna inductively coupled to the fixedstation, the portable object comprising a variable load impedanceconnected to terminals of the antenna, and a regulation loop regulatingthe voltage at terminals of the load impedance comprising rectifiermeans for rectifying the voltage at the terminals of the antenna andcontrol means designed to modify the load impedance according to theoutput voltage of the rectifier means, object characterized in that, thefixed station emitting an alternating magnetic field modulated inamplitude on 2^(N) levels by data to be transmitted, encoded on N bits,the portable object comprises demodulation means comprising an n-bitanalog-to-digital converters, clocked at an oversampling frequency muchhigher than the frequency of the data and connected between therectifier means and the control means, and digital processing meansconnected to the output of the converter.
 2. Device according to claim1, wherein the respective values of N and n are such that N≧2 and n<N.3. Device according to claim 1, wherein the digital processing means(comprise digital integration means supplying N-bit signalsrepresentative of the transmitted data.
 4. Device according to claim 1,wherein the control means comprise integration means supplying an analogvoltage signal representative of the n-bit output signals of theconverter.
 5. Device according to claim 4, wherein the integration meanscomprise a low-pass filter.
 6. Device according to claim 1, wherein n=1.7. Device according to claim 6, wherein the converter is formed by acomparator.
 8. Device according to claim 1, wherein N=2.
 9. Deviceaccording to claim 1, wherein the variable impedance comprises a MOStype transistor.
 10. Device according to claim 1, comprising amodulator, of sigma-delta type, connected between the input of theanalog-to-digital converter and an additional input of the digitalprocessing means.